Modern wireless networks support communication of diverse traffic types (e.g., voice, data, etc.) which have different latency requirements, with overall network and channel throughput requirements. Transmission time intervals (TTIs) with fixed lengths are not flexible for diverse traffic characteristics with different latency requirements.
In long term evolution (LTE) time division duplex (TDD), hybrid automatic repeat request (HARQ) feedback delay is variable, depending on the downlink (DL)/uplink (UL) configurations. A DL/UL configuration may be a pre-defined pattern of UL and DL TTIs over a radio frame, which may contain 10 TTIs. A DL/UL configuration provides a certain DL:UL traffic ratio. Based on the DL and UL TTIs' arrangements, a DL data transmission may be acknowledged in the next available UL TTI after taking into account the processing delay. A similar situation occurs for UL data transmissions. Thus, the HARQ feedback delay is not fixed. Also, due to the processing delay, the feedback may be longer than the fixed delay in the FDD system.
In Worldwide Interoperability for Microwave Access (WiMAX) TDD, every TDD frame (c.f. TTI in LTE) contains a downlink subframe and an uplink subframe. The duration of a TDD frame is fixed. Although the duration of the WiMAX TDD frame structure provides a fixed HARQ feedback delay, the existence of DL and UL subframes in the frames represents an overhead. It is desirable to have flexibility to accommodate different latency and dynamic signaling overhead based on different traffic characteristics.